fix: typo in setting LoginPolicy.IgnoreUnknownUsernames config (#11243)

# Which Problems Are Solved
The config `DefaultInstance.LoginPolicy.IgnoreUnknownUsernames` is
ignored during application start.

# How the Problems Are Solved
Fix the typo in the field name `IgnoreUnknownUsernames` associated with
the config used while unmarshaling.
Previously: `IgnoreUnknownUsername` 

# Additional Changes
N/A

# Additional Context
- Closes https://github.com/zitadel/zitadel/issues/11007

internal/command/instance_test.go

@@ -253,7 +253,7 @@ func instanceSetupPoliciesConfig() *InstanceSetup {
 			ForceMFA                   bool
 			ForceMFALocalOnly          bool
 			HidePasswordReset          bool
-			IgnoreUnknownUsername      bool
+			IgnoreUnknownUsernames     bool
 			AllowDomainDiscovery       bool
 			DisableLoginWithEmail      bool
 			DisableLoginWithPhone      bool